The role of RNA in gene regulation is often ignored. However, recent studies in metazoans have shown that the approximately 21nt RNAs that mediate RNA silencing (siRNAs) and the approximately 21nt endogenous RNAs (microRNAs) which act as translational regulators are both produced by a common set of proteins, homologs of the RNA selII enzyme Dicer and members of the Argonaute gene family, which are of unknown biochemical function. Dicer and Argonaute genes are highly conserved outside of metazoans, and mutations in these genes in Arabidopsis lead to floral and shoot meristem defects, making it likely that plants use endogenous small RNAs to regulate cell fate determinants in meristem development. In the last year, I have shown that microRNAs also exist in Arabidopsis and that the Dicer homolog CARPEL FACTORY is required for the accumulation of the Arabidopsis microRNAs. Prediction of miRNA regulatory targets in animals has been difficult because they are imperfectly complementarity to their target mRNAs. Arabidopsis miRNAs seem to display more extensive complementarity to their targets. We have identified several potential targets with known roles in meristem patterning, including two related transcription factors, PHABULOSA and PHAVOLUTA, which pattern radial cell fates in leaf primordia. The proposed research is designed to: 1) investigate the regulation of PHABULOSA and PHAVOLUTA by miRNAs; 2) identify PHABULOSA and PHAVOLUTA regulators and downstream effectors using a screen for genetic suppressors of gain-of-function phenotypes; and 3) identify new meristem patterning genes using candidate gene approaches.